1. Field of the Invention
The present invention relates to a cold cathode fluorescent lamp which is used as a light source of a liquid-crystal-display-use backlight device such as a liquid crystal monitor or a liquid crystal television receiver set, and more particularly to the bonding structure of a phosphor layer which is formed on an inner surface of a light transmitting glass tube.
2. Description of the Related Art
In general, a cold cathode fluorescent lamp is configured such that a phosphor layer is formed on an inner surface of a light transmitting glass tube, cold cathodes are arranged in the inside of the glass tube as electrodes, and a rare gas (also referred to as an inert gas) and a trace amount of mercury are sealed in the inside of the glass tube. The cold cathode fluorescent lamp radiates light when a high voltage is applied between electrodes arranged at both ends of the glass tube.
In an image display device which adopts a non-light-emitting liquid crystal display panel, an electronic latent image which is formed on the liquid crystal display panel is visualized by mounting an external lighting means on the liquid crystal display panel. As the external lighting means, except for the structure which utilizes a natural light, a lighting device is mounted on a back surface or a front surface of the liquid crystal display panel. Particularly, a display device which requires high brightness adopts the structure which mounts a lighting device on the back surface of the liquid crystal display panel as a mainstream. Such a lighting device is referred to as a backlight.
The backlight is roughly classified into a side-edge backlight and a direct backlight. The side-edge backlight is configured such that a linear light source represented by a cold cathode fluorescent lamp is arranged along a side edge portion of a light guide plate formed of a transparent plate. The side-edge blacklight is popularly used in a display device for a personal computer or the like which is required to satisfy the reduction of thickness. On the other hand, in a large-sized liquid crystal display device such as a display device used in a display monitor or a television receiver set, a direct backlight is popularly adopted. The direct backlight structure is configured such that the lighting device is arranged directly below a back surface of the liquid crystal display panel.
A general cold cathode fluorescent lamp which is used in such a lighting device is configured such that a pair of cold cathodes is arranged on both end portions of a light transmitting glass tube, a phosphor film is formed on an inner peripheral surface of the glass tube by coating, and mercury and a rare gas are sealed in the inside of the glass tube. Further, when a high voltage is applied between the cold cathodes arranged on both end portions of the glass tube, a discharge is generated in the inside of the glass tube so that ultraviolet rays having mainly a wavelength of 254 nm are generated due to the radiation by excitation of mercury. Phosphor is excited by the ultraviolet rays and radiates a visible light which forms an optical flux of emitted light.
In general, it is known that an optical flux maintenance factor of the cold cathode fluorescent lamp is gradually lowered due to lighting of the lamp for a long time. Along with the increase of an electric current which flows in the cold cathode fluorescent lamp for enhancing the brightness, such tendency becomes more apparent.
It is considered that the optical flux maintenance factor is lowered due to the following reasons. The first reason is that the transmissivity of the visible light of the light transmitting glass tube per se is lowered due to the coloration attributed to ultraviolet rays or the absorption of mercury. Further, the second reason is that the phosphor receives an impact of mercury ions at the time of lighting and hence, a surface of the phosphor is degenerated or mercury is adhered to and remains on the surface of the phosphor whereby a light emitting amount is decreased.
Accordingly, for preventing the deterioration of the light transmitting glass tube, generally, a glass tube which hardly deteriorates is used or a protective film made of material such as metallic oxide is formed on an inner surface of the glass tube.
On the other hand, for preventing the deterioration of phosphor, a technique which suppresses the deterioration of phosphor by forming a phosphor layer after forming a continuous film made of metallic oxide on phosphor powder is disclosed in JP-A-7-316551 (patent document 1). Further, a technique which suppresses the deterioration of the phosphor and enhances an optical flux maintenance factor by using a bonding agent which is obtained by mixing metallic borate and aluminum oxide particles which are hardly adhered or bonded to mercury is disclosed in JP-A-1-21856 (patent document 2).